1. Statement of the Technical Field
The invention concerns wireless communications equipment, and more particularly, frequency hopping adaptive base stations.
2. Description of the Related Art
A wireless cellular telecommunication system can comprise a base transceiver station (BTS) with an adaptive antenna system. A BTS can communicate with mobile units, such as mobile telephones, through RF links. There are many types of adaptive antenna systems, for example a frequency hopping (FHOP) adaptive base transceiver station. A FHOP adaptive base transceiver station is a BTS that comprises the features of both a FHOP BTS and an adaptive BTS.
A FHOP BTS rapidly changes the frequency of a transmission of the FHOP BTS. A FHOP BTS “hops” from frequency to frequency over a wide band. A code sequence within a FHOP BTS determines the order in which the frequencies are occupied. For example, frequency hopping software can provide a list (hop list) of frequencies for hopping such that a fixed pattern of a given number of frequencies at a given time is performed in repetition.
An adaptive BTS using an adaptive antenna array can increase user capacity over traditional antenna technology by amplifying the signals coming from and going to a mobile unit while dampening other signals coming from sources disposed in other directions. This ability is commonly referred to as “digital beam forming.” By steering a beam and positioning multiple nulls, an adaptive array is able to reduce co-channel and adjacent channel interference. This allows each cell to use all frequencies within an operator's licensed band and may even make it possible to use single carrier frequencies more than once within a given cell. An adaptive array separates the multiple signals having the same frequency provided the signals arrive from different angles or otherwise have distinctive propagation paths. In the same manner, multi-path arrivals of a desired signal, which in typical systems degrades signal quality due to inter-symbol interference, is used to define the spatial signature, thus isolating and enhancing the signal from interferers.
An antenna array consists of N identical antenna elements arranged in a particular geometry. The geometry of the array determines spatial resolution of the signals transmitted or received, i.e. the amount of coverage in a given spatial region. Commonly used array types are the uniform linear and circular arrays.
For any given geometry, the phases and amplitudes of the currents exciting the array elements as well as the number of array elements determine the gain of the array in a certain direction. The phases and amplitudes of the currents on the antenna array elements can be electronically adjusted such that received signals from a certain direction add in phase, and maximum gain is achieved in that direction. Due to the reciprocal nature of adaptive antennas, this approach is also generally applicable to focus the direction of transmitted energy from the antenna array for transmission as well.
In adaptive processing, the amplitude and phases of the individual array RF signals can be adjusted. To adjust the amplitude and phases of the individual array RF signals, complex weighting parameters can be used to modify the phase and amplitudes of each RF path of the adaptive array. The weighting parameters can be updated. Weighting parameter updating is usually accomplished adaptively to satisfy a chosen optimization criteria. There are several commonly used adaptive algorithms available for updating the weighting parameters. These include gradient based algorithms, recursive methods, and other such as the constant modulus method (CMA).
The adaptive array requires transceiver apparatus chains for operation. A transceiver apparatus chain can include an antenna element, a “receive apparatus chain,” and a “transmit apparatus chain.” Thus, each antenna element is provided a dedicated “receive apparatus chain” and “transmit apparatus chain.” For example, a “receive apparatus chain” may include cables, filters, RF electronics, physical connections, and an analog-to-digital converter, assuming the processing is digital. Likewise, each antenna array element along with its corresponding cables and the corresponding transmit electronics from the respective digital signal processor to the respective antenna elements shall be referred to as the “transmit apparatus chain” for each antenna element.
The BTS can include an adaptive array with two or more RF traffic transceiver apparatus chains and a RF beacon transceiver apparatus chain. As described above, the transceiver apparatus chains can include an antenna element, a receive apparatus chain, and a transmit apparatus chain. A radio signal can be transmitted at a traffic frequency from each RF traffic transceiver apparatus chains. If the BTS is designed for time division multiple access, the traffic frequency can be divided into a plurality of time slots. For example, Global System for Mobile communication (GSM) based systems typically divide a traffic frequency into eights time slots that are commonly referred to as burst periods. Each time slot can provide channel information to transmit communication traffic.
The RF beacon transceiver apparatus chain can be similar to the RF traffic transceiver apparatus chains. A radio signal can be transmitted at a beacon frequency from the RF beacon transceiver apparatus chain. For example, a GSM specification can define certain information communicated by the BTS on a beacon frequency. Typically, the beacon frequency is a frequency other than the traffic frequency. Similar to the traffic frequency used by a BTS designed for time division multiple access, the RF beacon frequency can be divided into a plurality of time slots. For example, Global System for Mobile communication (GSM) based systems typically divide a beacon frequency into eight time slots. The first time slot of the beacon frequency can provide given channel information to allow a mobile unit access to the BTS. The other time slots of the beacon frequency can provide channel information to transmit communication traffic.
A BTS requires calibration of the adaptive antenna system to compensate for RF path variations due to environmental conditions. The adaptive antenna system will have calibration values for radio signals transmitted from the BTS. It should be noted that the phase and amplitude shifts that occur in the transmit apparatus chains are, in general, frequency dependant. Thus, in broadband applications, calibration values must generally be determined at the plurality of carrier frequencies used, or at least a sampling of these frequencies.